The field of the present invention is processes for the separation of elements.
Recovery, separation and purification of valuable precious metals, e.g., platinum [Pt], palladium [Pd], iridium [Ir], rhodium [Rh], ruthenium [Ru] and gold [Au], are typically tedious processes requiring repeated application of pyrometallurgical, hydrometallurgical or electrowinning processes to achieve acceptable metal recovery and metal purity. Most source materials such as ores, spent catalysts, plating solutions, sludges, ore concentrates and smelter mattes are chemically complex, not only because of the diversity of precious metal elements but also because of the presence of large quantities of non-precious metals in the source materials. Recovery, purification and separation of precious metal elements from these source materials are very difficult, time consuming, expensive, and unsatisfactory with respect to some elements, particularly rhodium.
Precious metal chemistry is exceedingly complex and not always well understood. In acidic halogen media, more specifically chloride media, precious metals do not form simple aquated cations of the type formed by common base metals (e.g., Ni(H2)+2). Precious metals form coexisting coordination complexes with a wide variety of ligands. For example known rhodium (III) complexes include [RhCl6]−3, [RhCl5(H2O)]−2, [RhCl4(H2O)2]−2 and [RhCl3(H2O)3]. In 1M chloride solution, an equilibrium assemblage comprises about 80% [RhCl5(H2O)]−2, 10% [RhCl6]−3, 10% [RhCl4(H2O)2]−2 and less than about 1% [RhCl3(H2O)3] (See Grant, R. A., 1989, The Separation Chemistry of Rhodium and Iridium, in Manziek, ed., Precious Metals Recovery and Refining, Proceedings of the International Precious Metals Institute, p. 7-41). However, the distribution of coexisting complexes is unlikely to be an equilibrium assemblage, particularly in industrial applications involving multiple precious metals. The kinetics of ligand exchange rates for precious metals exhibit extreme ranges. For example, the relative exchange rate for platinum (IV) is 10−12 relative to palladium. Whereas palladium could undergo ligand exchange within hours or days, platinum (IV) ligand exchange may take months. It is likely that combined complex precious metal speciation, reaction kinetics and other factors have necessitated development of present-day complex, tedious and time-consuming precious metal recovery protocols that often suffer from disappointing outcomes or expense.
Current industry practice for precious metal recovery relies chiefly on solvent extraction that is typically supplemented by ion exchange and traditional chemical processes in spite of inefficiencies, long processing time, large volumes of solutions and a myriad of other problems. U.S. Pat. Nos. 5,201,942 and 7,291,202 review some of the difficulties related to precious metal recovery and purification. Precious metal recovery and purification processes in these patents as well as U.S. Pat. No. 7,175,818 disclose tedious and complex processes that do not fully solve current limitations or high costs.
The precious metal industry has exerted great effort over the past decades in developing and improving upon solvent extraction processes for gold and some platinum group metals. In spite of substantial progress, protocols typically utilize different extractants for each element. Solvent exchange processes are equilibrium controlled. Each solvent extraction may require multiple contacts of extractant and mother liquid, stripping of the extractant, scrubbing of the extractant to remove impurities and regeneration of the extractant. Therefore, complete extraction of the desired metal may not be achieved and cumulative metal loss can increase with each extraction. In addition, contaminating elements may also be extracted along with the desired metal, thus requiring purification of the desired metal. Solvent extractions, while the method of choice, may be complicated further by poor solvent-mother liquid separation, solubility of the extractant in the mother or scrub solutions, multiple side streams of the desired metal, large volumes of aqueous solutions requiring treatment as well as toxicity or flammability of the organic extractant.
Ion exchange processes have been selectively incorporated into industrial precious metal recovery protocols but are of more limited use than solvent extraction processes and do not solve recovery and refining challenges. Ion exchange processes are typically equilibrium controlled and may not completely remove the desired metal. Ion exchange processes may also suffer from poor selectivity for precious metals which result co-extraction of other precious metals or from co-extraction of contaminants. Typically, ion exchange resins for precious metals are expensive, have relatively low capacity thus requiring large volumes, require large volumes of strip solutions to recover the desired metal and extracted metals often require additional purification.
Other chemical processes of precipitation and volatilization may be used in conjunction with solvent extraction and/or ion exchange. Typically, these processes are used to remove selected elements from the mother liquid or to purify metals of insufficient purity.
Separation and recovery of rhodium is a particularly vexing problem. In typical current extraction processes, rhodium remains at the end after recovery of other platinum group metals. This has at least three disadvantages: rhodium is lost in the many sidestreams of upstream extractions, rhodium is locked up during recovery of other precious metals, and rhodium and iridium are often in the final solution and separation of these two metals is difficult by any known method.
In spite of much effort, rhodium recovery and refining remain a vexing problem. There are no known readily usable solvent extractants for rhodium. U.S. Pat. No. 5,201,942 proposed a solvent extractant for rhodium requiring complexing of Rh with a high molar excess of tin. The proposed solvent extraction process has distinct limitations: it presumes osmium and iridium have been removed by other methods, rhodium extraction is not quantitative and Rh must be further processed to remove tin which may be in a 10 times molar excess. Ion exchange processes for Rh recovery and purification on an industrial scale have likewise returned disappointing results or have been unsuccessful. By necessity, the industry largely utilizes tedious precipitation and dissolution methods to purify rhodium.
To meet the demand for precious metals, the industry is in need of a simpler, lower cost alternative to current practices that is capable of recovering precious metals from chemically diverse primary and secondary sources.
All the U.S. Patent references described above are incorporated by reference in their entirety for all useful purposes.